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Caster's True Name is Thomas Alva Edison (トーマス・アルバ・エジソン, Tōmasu Aruba Ejison?), the King of Inventors (発明王, Hatsumei-Ō?) and lifelong rival and nemesis of Nikola Tesla. Born in the 19th century, he was an American inventor whose vast number of achievements are such that they need not be said. He was one who, rather than coming up with inventions on his own, was "extremely proficient at reconstructing the work of other pioneers into easily proliferated forms." He was extremely dedicated to his work, sleeping only three to four hours per day and otherwise fully exerting himself in work at all times. He held cordial relationships with many other inventors, such as Alexander Graham Bell and Charles Babbage, but his rivalry with Tesla was the one exception.

Although granted a lion's head due to the summoning, Caster's intellect is undiminished, allowing those familiar with him to realize "ah, this is indeed Edison" after a bit of conversation. He has a very straightforward personality, and it is said there are not many points of caution for the Master when dealing with him as a Servant. He does unconsciously repeat the work habits he had in life, so careful attention is recommended to the Master so as to keep him from sleep deprivation or overwork.

Due to his rivalry in life with Tesla, they will have all relations inevitably break down into a fistfight upon both manifesting as Servants.

Edison is summoned in the "E Pluburis Unum" Singularity. To combat the Celtic army of Medb, the past American presidents improve his abilities with a Mystic Code, although their combined ambitious natures cause Edison to become more tyrannical. Edison manufactures an army of robots to combat the Celts, becoming the President-King of the United States, while also allying with Helena Blavatsky and Karna. Edison intends to get the Holy Grail from Medb and use it to speed up the development of the era.

The Protagonist's party are captured by Edison's forces and brought to his castle, where Edison tries to convince them to join him. The party refuses his offer, causing Edison to imprison them. However, they escape, thanks to the help of Geronimo, a member of the Resistance that opposes both the Celts and Americans. The Protagonist's party later face Edison again, infiltrating his castle and defeating him. They then manage to form an alliance with him, unifying their forces against the Celts. They split their forces into two armies that attack the base of the Celts, with Edison being one of the leaders of the northern army. The northern army confronts the Celtic warriors, with Edison helping in the battle. When Nikola Tesla arrives to help the northern army, Edison is annoyed by his presence and battles him after the threat of the Celts is neutralized.

Caster would normally be a weaker Servant with parameters and Class Skills ranging from D to E. Despite his considerable renown, he is one of the most recent Heroic Spirits, so his power as a Servant is lacking. For this summoning, all the Presidents of the United States of America throughout history granted him a Mystic Code that "would reinforce the concept of 'Edison'" to compensate for this weakness. All of his abilities have been augmented, making for a strange distribution of his parameters and Class Skills where they are either E or EX, aside from Luck. His Noble Phantasm is World Faith Domination. Caster also has a superhuman elixir that, if imbibed, can turn him into Thomas Mazda Edison; however, in exchange, Caster will lose his humanity.